The present invention relates generally to guided wave optics, and, more particularly, to relatively high-power laser light sources for applications such as optical communication systems. For most types of optical communication systems it is advantageous to employ a laser light source of high power output. For a given distance of transmission, a higher power will result in a higher signal-to-noise ratio at the receiving end. Likewise, for a given signal-to-noise ratio at the receiving end, a higher power will result in a longer transmission distance. Semiconductor injection lasers are frequently chosen as the light sources in optical communication systems, principally because of their small size and reliability.
In a semiconductor injection laser, a forward voltage bias is applied across a p-n semiconductor junction and minority current carriers are injected across the junction. Initially, at low current, there is spontaneous emission of photons in all directions in an active region of the junction. As the current increases, a threshold is reached at which stimulated emission occurs and a monochromatic, highly directional light beam is emitted from the active region. The active region is bounded at opposite ends by cleaved crystal facets, which serve as laser mirrors, and by roughened side surfaces to prevent emission in lateral directions. The power output of such a laser is limited by two factors. First, there is an upper threshold of power density beyond which there will be catastrophic damage or degradation of the laser performance, principally due to either the pitting of the crystal facets serving as laser mirrors or the formation of dark line defects. Power is also limited by the finite cross section of the active region of the laser.
By using a relatively large laser cavity, one can obtain a laser output of approximately 60 milliwatts (mW) for continuous wave (CW) operation at room temperature. While further improvements may be feasible, it is not expected that semiconductor lasers will be able to provide outputs in excess of 100 mW. Coupled laser arrays have recently achieved considerably higher output powers, by the use of waveguide coupling of laser cavities. However, the combined output of such an array has a relatively large aspect ratio, at least in its near-field pattern, and special lens systems are needed to provide a circular field pattern. Moreover, if the degree of coupling between the laser cavities is not strong enough, the phase front of the combined outputs will not be planar. This phase front distortion may not be a problem in a multimode fiber communication system, but would result in a lower useful power in some optical communication systems, such as single-mode fiber systems and optical space communications.
A coupled array of semiconductor lasers is shown in U.S. Pat. No. 4,255,717, issued to Scifres et al. A number of spatially displaced laser cavities are coupled together by any of a variety of disclosed techniques, apparently for purposes of increased power output, and improved coherence and divergence of the emitted light pattern. In all of the disclosed embodiments, however, the output is in the form of an array. There is no improvement in output power density. In addition, no consideration is given in the patent to producing a single output of circular cross section.
U.S. Pat. No. 4,309,667 issued to Di Forte et al. and U.S. Pat. No. 4,318,158 issued to Mito et al. also disclose coupled laser arrays. However, in both instances the arrays are coupled for purposes of combining laser outputs of different frequencies, in wavelength-division multiplexing systems.
It will be apparent from the foregoing and from a review of the prior art discussed above, that there has been a need in the guided wave optics field for a semiconductor laser structure that is capable of producing a single, coherent, high-power beam with low divergence and no wave-front distortion. The present invention satisfies this need.